Method of sealing anodized aluminum



United States Patent 3,365,377 METHOD OF SEALING ANODIZED ALUMINUM ABSTRACT OF THE DISCLOSURE The present disclosure teaches an improved method for sealing anodized oxide coated aluminum characterized by increased hydration over normal sealing. The process aqueous solution containing triethanolamine and maintained at an elevated temperature.

The present invention relates to a method of sealing anodized aluminum or aluminum base alloy surfaces. More specifically, the present invention relates to a method for sealing an anodized oxide coating formed on aluminum surfaces.

For many purposes, an oxide layer is formed as a protective layer on aluminum surfaces 'by making the aluminum the anode in an electrolytic cell having an electrolyte formed of about 2 to 70 percent by weight sulfuric acid, or other acids or acid salts of the type chromic acid, oxalic acid, sulfamic acid, and the like. Any suitable metal, such as lead, for example, may form the cathode. A voltage of about 10 to 20 volts is impressed upon the cell while the electrolyte is held at a suitable temperature, such as from 10 to 50 C. until an oxide coating of the desired thickness is formed, generally from 5 to 60 minutes or more depending upon the thickness of the coating desired.

In addition to the foregoing, there are a number of less commercial processes for producing oxide layers on aluminum.

The oxide layer formed under these conditions consists generally of relatively anhydrous alumina, A1 0 containing sulfate ion. Especially when formed by anodic oxidation, the layer of aluminum oxide is relatively hard, porous, highly absorbent and of substantial thickness, depending upon the particular aluminum sample and the specific process for forming the oxide coating.

The introduction of an oxide coating on the aluminum surface is intended to improve resistance to corrosion, resistance to abrasion and absorption of coloring to provide permanent color.

It had been found heretofore that the characteristics of the oxide film can be markedly improved by a process hereinafter referred to as sealing, for rendering the film impervious and less porous. Sealing is generally carried out commercially by immersing the anodized aluminum in Water maintained near its boiling point, i.e., Within about F. of the boiling point. Sealing is believed to consist primarily in the conversion of part of the substantially porous and pervious oxide film of anhydrous alumina to a hydrated product, such as aluminum monohydrate, Al O -H O, with consequent swelling or volume increase of the oxide particles to partially close or seal the pores.

To improve corrosion resistance, the sealing bath has been modified by various additions, such as chromic acid or boric acid or metal'salts, such as nickel acetate, and others. These modified processes offer some improvement, but optimum conversion is not achieved.

It is, therefore, the principal object of the present ininvolves immersing the oxide coated aluminum in a dilute Vention to provide a method of sealing an anodized oxide coating formed on aluminum.

It is a still further object of the present invention to provide a method as aforesaid which simply and effectively attains improved hydration of the aluminum oxide coating.

Further objects and advantages of the present invention will appear hereinafter.

In accordance with the present invention it has now been found that the foregoing objects and advantages may be readily accomplished and a method of sealing an anodized oxide coating formed on aluminum or aluminum base alloys may be attained by immersing the oxide coated aluminum in a dilute aqueous solution containing triethanolamine for at least 5 minutes, said solution being maintained at a temperature near its boiling point within about 10 F. thereof, e.g., in the range of about 200- 212 F.

The process of the present invention attains marked and increased hydration over normal sealing in boiling water or over normal sealing in boiling water containing nickel acetate, for example.

The process of the present invention is readily applicable to any aluminum or aluminum base alloy in accordance with prior practices. Exemplificative aluminum base alloys which may be readily used include alloys 1100, 3003, 5453, 5053, 5052, 6061, 6063, etc.

The oxide coated aluminum is immersed in a dilute aqueous solution containing triethanolamine for at least 5 minutes, with the aqueous solution being maintained near its boiling point, e.g., in the range of about 200- 212 F. The exact treatment time will naturally vary with coating thickness, but it is generally in the range of from 5 to 30 minutes. The concentration of the triethanolamine in the aqueous solution may vary over a wide range, for example, from 0.003 N to 0.02 N. Generally, it is not desirable due to economics to use excessive quantities of triethanolamine. Hence, the preferred range of triethanolamine is from 0.003 N to 0.007 N. Naturally, noninterfering additives may be employed.

In accordance with the present invention it has been found that the simple and convenient process thereof attains a marked and surprising increase in the amount of hydrate formed over that obtained in the same amount of time of sealing using prior art practices.

The improvements obtained in accordance with the present invention will be more clearly understood from a consideration of the following illustrative example.

EXAMPLE Samples of aluminum alloy 5257 were anodized for 15 minutes at 12 amps per sq. foot in a 15 percent by weight sulfuric acid solution maintained at 22 C. The samples were bright dipped in a mixture of 5 percent nitric acid and the balance phosphoric acid before anodizing. After anodizing, the samples were rinsed and sealed for 12 minutes in boiling water containing triethanolamine as shown in the table below which shows (1) the concentration of the triethanolamine and (2) the resultant infrared absorption at 2.9 microns:

Table Triethanolamine Hydration-infrared concentration: absorption 0 0.55 0.02 N 0.80 0.05 N 1.20 0.0067 N 0.73 0.0067 N 0.87

. The above table shows a significant increase in the infrared absorption peak at 2.9 microns, indicating a significant increase in the degree of hydration of the oxide.

This invention may be embodied in other forms or carried out in other ways without departing from the spirit or essential characteristics thereof. The present embodiment is therefore to be considered as in all respects illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and all changes which come within the meaning and range of equivalency are intended to be embraced therein.

What is claimed is:

1. The process which comprises:

(a) providing an aluminum containing material selected from the group consisting of aluminum and aluminum base alloys;

(b) forming an oxide layer on said aluminum containing material by electrolytically anodizing said aluminum containing material; and

(c) sealing said anodized oxide layer by immersing the oxide coated aluminum containing material in a dilute aqueous solution containing monomeric triethanolamine in a concentration of from 0.003 N to 0.02 N for at least 5 minutes, said solution being maintained at a temperature within 10 F. of its boiling point, thereby obtaining increased hydration of said oxide layer. 2. A process according to claim 1 wherein said temperature is from 200 to 212 F.

3. A method according to claim 1 wherein said anodizing oxide coated aluminum containing material is im- 0 mersed in the dilute aqueous solution for from 5 to 30 minutes.

References Cited UNITED STATES PATENTS 3,026,255 3/1962 Riou et al. 20433 3,210,184 10/1965 Uhlig 96-1 HOWARD S. WILLIAMS, Primary Examiner.

JOHN H. MACK, Examiner.

20 W. VAN SISE, Assistant Examiner. 

